var str = 'abc'  // new String('abc')
str += 1   // str = 'abc1'
var test = typeof(str)   // 'string'
if (test.length === 6) {
  test.sign = 'typeOf的返回结果可能是String'
  // new String(test).sign = 'xxxxx'
  // delete test.sign
}
console.log(test.sign); // new String(test).sign